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Tuvia Liran,Nano Retina
Citation preview
May 2, 2012 1
Practical issues and solutions in
Ultra Low Power designfor Artificial Retina
May 2, 2012
Tuvia LiranNano Retina Inc.
May 2, 2012 2
Outline
• What is Artificial Retina• Key technical challenges• Micro-architecture of Artificial Retina• Challenge #1 – power reduction• Challenge #2 – selection of supply source• Challenge #3 – controlling very low bandwidth• Summary
May 2, 2012 3
Vision Pathway with Bio-Retina
Optic nerve
Macula
Retina
Bio-Retina glass
Bio-Retina Implant
Ganglion Bipolar Photoreceptors
Nano Retina Confidential3
What is artificial retina
May 2, 2012 4
• Bio-Retina implant includes:
– Receives visual images
– Converts the image into neuron stimulation
– Operates from infrared radiation power source
– Configured by wireless optical transmission
• Eyeglasses includes:
– Infrared power source
– Rechargeable battery
– Implant control
Artificial Retina technology byNano Retina
IR laser source
Bio-Retina Implant Retina
IR laser beam
Advantages: Light and long lasting Simple implantation Uses the eye’s natural optics
and nerves
May 2, 2012 5
Nano Retina technology
5
May 2, 2012 6
Key technical challenges • Ultra low power (ULP) (<300uW)
• Weak power source• Very low bandwidth (1÷40Hz)
• Wide dynamic range (>5 decades = ~17 bits)
• Sufficient resolution (~1000 pixels)
• Wirelessly configurable (w/o antenna)
• Very small device (<15mm2 including package)
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May 2, 2012 7
• Phase 1: Providing proof of concept– Development of Micro-electrode array
(MEA)– Defining process flow & vendors– Development of ULP test chip– Performing pre-clinical experiments
• Phase 2: Development of prototypes– Fully compliance for medical experiments– Full functionality– Complementary accessories
Key development steps
May 2, 2012 8
Micro architecture of Artificial Retina
• MEA: by NR• Design: CSEM
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• Process: TSL018 CIS• Power: GaAs PV
May 2, 2012 9
Challenge #1: Very low power• Why it is important?
– Ability to transfer optical power without exceeding eye safety limits– Minimizing heating of the eye– Minimizing the batteries on the glasses
• Considering the use of regulator– Regulator consume significant portion of total power– Regulator reduces the usable voltage swing– Regulator is a must when AC power source is AC or unstable– Regulator reduces the performance variation due to supply voltage
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May 2, 2012 10
How it is achieved?• ULP oriented micro-architecture
– Minimum usable and achievable voltage– Minimum digital processing– Minimum frequency at any stage– Discrete time analog processing
• ULP analog design style by ULP experts• Low voltage operation
10
ULP design techniques are enablers for implementing medical implantable devices,
such as Artificial Retina
May 2, 2012 11
Challenge #1a – sub-threshold circuits
• Vdd ~Vth -> operation at sub-threshold region• Selecting proper circuits:
– Dynamic range– Power– Linearity– Matching– Noise
May 2, 2012 12
Operating CMOS in sub-threshold
• Operating in sub-threshold (weak inversion)
• Voltages are scaled to nUT (32 mV for n = 1.2 )
• ∆Id/∆Vgs = 70÷80mV/decade • Weak inversion expression:
T
TGsDsat nU
VVII 0exp
C37at mV 7.26 Tq
kTUT
CSEM Microelectronics
May 2, 2012 13
Example of sub-threshold low voltage circuit – ULP amplifier
Presented by E. Vittoz
Amplifier with controlled offset
Amplifier with extended
dynamic range
May 2, 2012 14
Challenge #1b – device modeling
• Conventional BSIM3/4 are not accurate at sub-threshold
• EKV models are more accurate (availability ?!)• Need also accurate models for monte-carlo & noise
May 2, 2012 15
Challenge #1c – mismatch in sub-threshold circuits
• Mis-matching is high• Mitigation techniques:
– Large devices– Very careful layout – matching rules– Dynamic offset cancellation (chopping):
Presented by C. Enz & G. Themes
May 2, 2012 16
Challenge #2 – Selection of power source
• Relevant power sources:– Battery– Energy harvesting– Electromagnetic power– Optical power transfer
May 2, 2012 17
Performance limitations of PV
• Limited selection of voltages• Limited current• High dV/dI• Fast voltage drop at over-
loading
Laser IR transmitter + Photo-Voltaic receiver is the preferred power source for Artificial Retina. But…
May 2, 2012 18
Specifying voltage requirements
• Analog circuit requires higher supply voltage than digital
• Limited by dynamic range of analog circuits– Example: Logarithmic Trans-Impedance Amplifier
May 2, 2012 19
Challenge #3 – controlling very low bandwidth
• Bandwidth of neural signals is <<100Hz– RC example: C << 10pF ; R >> 10Gohm (!!!)
• Analog implementation of high giga-resistor:– Transistor at very weak inversion– Linearity ?!– Immunity to leakage ?!– Matching ?!
• Switch capacitor implementation:– CMOS switches– Charge injection !!!
May 2, 2012 20
Implementing ULP switch capacitor resistor
• General SC resistor– Minimum size CMOS switches– Capacitors implemented by:
• NMOS• MIM• Native_NMOS
• Charge injection:– Might cause offset– Difficult to predict– Difficult to match
• Differential SC circuits are better
May 2, 2012 21
Summary• ULP is enabling technology for implantable medical devices,
such as Artificial Retina• ULP implementation is challenging but doable• Key factors for successful design:
– Optimal micro-architecture– Optimal selection of power sourcing– Know-how in sub-threshold design– Availability of EKV models– Intensive monte-carlo simulations
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